I personally feel Fallout 1 is the best, but its all down to personal preference. Fallout 1 is great for its story and its setting but you could say it was kinda meh in the gameplay department (Still good mind you).
Fallout 2 is great for its gameplay improvements and expanded setting, but many dislike it because of its zany style and pop-references.
Fallout Tactics is a great spin-off featuring good squad-based turn-based (And Real Time too) tactics, but some dislike it because it isn't a rpg like the previous two games.
Fallout: Brotherhood of Steel is something many won't mention, and for good reason.
And finally Fallout 3, great for its immersion, graphics, and accessibility, but lacking in story, depth, and gameplay (Arguable depending on whose saying.)
Edit: Forgot the main question: Buy 1 and 2 if they interest you. If you don't mind isometric, turn-based rpgs that tend to be on the hard side (And if you don't mind bad/not fancy graphics)